1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copier or printer, and more particularly to a device and method to detect the amount of toner on the image carrier of such apparatus, as well as the color shift amount.
2. Description of Related Art
In an electrophotographic image forming apparatus of the conventional art, such as a copier or printer, multiple photoreceptors are disposed side by side such that they face a belt that is driven to rotate, and toner images of different colors are formed on the photoreceptors and then sequentially transferred onto the belt in order to obtain a color image.
In such an apparatus, in order to form toner images on the photoreceptors, various devices including a charging device, developing device and exposure device are disposed around each photoreceptor. In order to optimize the output of each such device, multiple toner pattern images formed in accordance with different image formation parameter values are formed on each photoreceptor and transferred onto the belt. The amount of toner for each toner pattern image thus transferred is detected using a toner amount detection sensor (hereinafter an ‘AIDC sensor’) disposed such that it faces the belt, based on the result of which the output of each of the above devices is adjusted.
In addition, in the above apparatus, toner images of multiple colors are overlaid on top of one another in order to form a color image. Consequently, transfer of any of the various color toner images at an incorrect position results in color shift, making the final color image less than ideal. In order to prevent this from happening, according to the conventional art, detection patterns of each of the various colors (hereinafter ‘resist patterns’) are formed on the belt to detect the position of the toner image of each color prior to the actual image forming operation, and a shift in the position of a detection pattern for a given color, i.e., color shift, is detected using a resist sensor.
In recent years, focusing on the fact that the AIDC patterns and resist patterns described above are both formed on the same belt, a device that functions as both an AIDC sensor and a resist sensor has been considered, enabling resist patterns to be detected using an AIDC sensor.
However, the problem exists that AIDC sensors were originally developed to measure image darkness, and therefore cannot accurately detect a color shift amount.